In the process of manufacturing semiconductor wafers it is typically necessary to deposit layers of various materials onto a silicon wafer substrate to create desired electrical properties. Processes such as epitaxial deposition (EPI) or chemical vapor deposition (CVD) are commonly used to deposit materials onto substrates.
The properties of the layers deposited on the wafers often depend on the thickness and uniformity of the layers, and minute variations in layer thickness may dramatically alter the properties of the semiconductors manufactured therefrom. Semiconductor fabricators must therefore measure and control the thickness and uniformity of the layers with great accuracy. Many methods of measuring layer thickness have been devised using technologies such as ellipsometry, heat reflectivity, interferometry, and infrared reflectance. One commonly used measuring method is Fourier Transform Infrared Spectroscopy (FTIR). In this method, a silicon wafer acts as a shoulder of Michelson interforometer. The incident IR beam is split and one part of the radiation reflects from the wafer, while the other reflects from a moving mirror. These two parts of the radiation are then reflected to a detector where they produce an interference signal. Epitaxial layer thickness is calculated from the distance between interference maxima reflected from the substrate-layer interface and top silicon surface. This process may be repeated several times at different places on the wafer to ensure that the layer is uniformly deposited on the wafer.
In a semiconductor manufacturing process, it is typical to deposit a layer of material on a plurality or batch of silicon wafers at the same time. To conserve time, one wafer per batch may be tested using FTIR and the results of the test are assumed to apply to all the wafers in the batch. More regular testing, such as testing every fifth or tenth wafer, is also possible, but the more regular the testing, the slower the process becomes. Testing every wafer using FTIR may be too time-consuming for some manufacturing processes.
FTIR has other limitations. For example, the level of dopant concentration in the substrate must be three to four orders of magnitude greater than the level of dopant concentration of silicon in the deposited layer so that enough radiation can be reflected from the substrate-epitaxial layer interface. In addition, FTIR is effective when the thickness of the layer to be measured is no less than two microns. FTIR measurement of layer thickness and uniformity may not be practical for semiconductor designs that require either relatively equal levels of dopant concentration of silicon or the deposition of very thin layers. Furthermore, FTIR and the other technologies enumerated above require the purchase and maintenance of expensive and complex equipment.